An additive method is known as a method for formation of a circuit of a printed wiring board. In accordance with the additive method, forming resists for plating on a seed layer of a substrate composed of an insulator substrate having a conductive layer (the seed layer) uniformly formed on one side thereof, and forming plating portions respectively at intervals between the resists by electrolytic plating are carried out. After this, by removing the resists for plating and etching and removing the seed layer among the plating portions, a conductor circuit is obtained.
As compared with formation of a conductor circuit by a subtracting method (or an etching method) in which a circuit is formed by etching an conductive layer composed of copper foil uniformly formed on an insulator substrate, the additive method is known to be capable of forming a circuit of thinner linewidth. In view of the state that down-sizing or densifying electronic devices is required, the additive method has a great advantage in this point.
However, the conventional additive method has the following demerits. More specifically, because of occurrence of diffused reflection and such by the seed layer at a time of exposure by photo lithography, bottom portions of resists 103 as a boundary layer with a seed layer 102 come to have base portions 104 widening toward the bottom portions as shown in FIG. 4A. Plating with respect to this state is carried out as shown in FIG. 4B, thereby bottom portions of plating portions 105 have undercut portions 106 as being gouged as shown in FIG. 4C.
After this, though etching is carried out as described above, the undercut portions 106 are eroded more deeply as shown in FIG. 4D. An insulator coating such as a cover lay material or a solder resist material cannot be sufficiently filled in the deep undercut portions 106. It gives rise to possibility to leave gaps therebetween. The gaps may cause migration or such.
Moreover, if the undercuts portions are generated, because a contact area between the conductor circuit and the insulator substrate becomes smaller, an adhesive strength therebetween decreases. It may give rise to possibility that the conductor circuit peels off by a heat treatment. Furthermore, because a cross sectional shape of the conductor circuit fails to be rectangular, a problem of impedance mismatch in high-frequency signal transmission may be induced.
Japanese Patent Unexamined Publication 2001-196740 discloses a method of forming a conductor circuit free from undercuts. In accordance with this method, after removing the base portions of the bottom portions of the resists by a dry treatment, plating for circuit formation is carried out so as to suppress occurrence of the undercuts.